Snow on Halloween? Millions under winter weather advisories, forecast says.

Millions of Americans could face wintry weather on Halloween, including snow in the upper Midwest and heavy snowfall in high elevations of the Pacific Northwest.

Meanwhile, the Eastern U.S. prepares to trick-or-treat in temperatures "reminiscent of summer," the National Weather Service said Thursday.

Moderate to heavy snow is expected across parts of the Southern Cascades and Northern Intermountain Region, as well as the upper Midwest, the weather service said. Several winter storm warnings and winter weather advisories that were placed earlier this week remain in effect.

"A Pacific weather system will bring some snow to the west that will turn the ground white as a ghost on Halloween in western Wyoming. A gusty, spooky wind will blow from Wamsutter to Casper at times," the weather service in Riverton, Wyoming said.

In total, nearly 4 million people live in areas under winter weather advisories on Thursday, according to the National Weather Service.

Those in the mountain ranges may see up to a foot of snow as colder air combines with high moisture, according to the weather service. Conditions may make traveling difficult, especially in the dark.

"Be prepared for winter driving conditions and slower travel should you need to venture out into the high country. In the wake of the storm, much colder air will lead to frost and freeze concerns for the lower elevations," the weather service warned on Wednesday.

Halloween snow for upper Midwest

On Halloween, moderate to heavy snow is likely over parts of Minnesota, extreme northern Wisconsin, and the western portions of the Upper Peninsula of Michigan, the weather service said.

The metro area most affected by the snow on Halloween will be the Minneapolis/St. Paul area, where the weather service said, "If you plan to travel Thursday, be prepared to encounter rapidly changing conditions."

By Thursday morning, rain was transitioning into snowfall in the Twin Cities, creating slushy conditions. Snowfall could exceed 1 inch per hour at times on Thursday, the weather service there said.

The chill will be widespread Thursday: Weather.com meteorologist Caitlin Kaiser said, "a winter coat will be a good addition to your Halloween costume if you live in the Northwest, Rockies or upper Midwest. Temperatures will plummet into the 40s, making evening festivities chilly."

(This story has been updated to add new information.)

This article originally appeared on USA TODAY: Snow in forecast for Halloween across mountains, Midwest